In the injection molding of plastic objects, the cavity formed from the walls or surfaces which define the shape of the object to be molded often has two or more openings or gates through which the plastic material enters the cavity. The several openings provide for better control of the cooling conditions around the gates and a more even flow of the injected material under less pressure to the extremities of the cavity.
However, utilizing several openings for the injection of the plastic imposes additional requirements upon the molding process. The necessity of maintaining a balanced flow of plastic throughout the cavity stands foremost amongst these. An unbalanced flow creates several problems. First, in a balanced flow the plastic flows generally along predetermined lines which, in objects having corners, compartmentalizing walls, and other irregular features, preclude the development of stresses in the molded product. Having more plastic than desired passing through some of the gates with less passing through the others results in an unbalanced flow. In such an unbalanced flow, the plastic approaches various locations in the cavity from undesired directions which, in the final object, will create stresses.
Second, injection-molding apparatus generally includes a cantilever-mounted core. Placing the core within the cavity half of the mold defines the cavity corresponding to the structure of the desired object. Further, with multicompartmented molded objects, such as battery cases, the core includes several elements each of which has a cantilevered mounting. An unbalanced flow of material through a cavity formed with cantilever-mounted cores, and more particularly several cantilever-mounted core elements, results in the exertion of considerable stress upon the core. This stress has the obvious effect of shortening the useful life of the mold assembly and may actually shift the core and result in loss of dimensional stability.
To help achieve a balanced flow of injected material through the cavity, use is often made of what is commonly called a "short shot". This technique simply involves the introduction into the cavity at different times of different quantities of material none of which suffice to completely form the desired object. The partially formed objects produced by these short shots reveal whether some portions of the cavity receive relatively greater amounts of material than others; if so the need exists to alter the relative amounts of material flowing through the gates. The balancing process can continue until the "short shots" show the achievement of a balanced flow of plastic within the cavity.
However, the short-shot process has generally found use only in adjusting the molding apparatus prior to an actual production run. Upon the achieving of a balanced flow and the placing of the apparatus in continual production, the short-shot technique no longer finds use. This occurs for the obvious reason that the short-shot process precludes the apparatus from producing useful parts which thus incurs an economic loss. If the molded part appears satisfactory, the apparatus remains in production use and does not undergo testing.
Further, the plastic used in a short shot must then be discarded or reground for reintroduction into the molding process. The former alternative represents a loss of material and the latter a waste of time.
As an alternate to the short shot to aid in the initial adjustment, one manufacturer used pressure transducers in various portions of a cavity which has only a single gate for the introduction of material. These transducers apparently played a role similar to that of the short-shot technique; the output from the transducers could show the need for a modification of the overall cavity structure in order to achieve a balanced flow of material to the various extremities of the cavity. The utility of this technique, of course, like the short-shot process, ceases upon the placement of the apparatus into the actual production of the molded parts.
The U.S. Pat. No. 3,473,197 to R. W. Wilds et al. describes a modified apparatus for relieving some of the strains incurred in molding apparatus with cantilevered core elements. As described below in connection with FIGS. 1-4, this apparatus includes wedges which lock the various elements into a set position during most of the molding cycle. Nonetheless, the uneven introduction of material into this improved type of structure will shorten the useful life of the mold assembly.